OClCollMeshEngn: Produce ambience count; set postrin threshold

We modify the semantics/meaning of the ambience stim feature.
It now represents the number of frames whose average intensity
is below the ambienceLowVal.

We can now implement the postrin as the event wherein the number
of frames whose intensity <= ambienceLowVal exceeds
postrin-interest-threshold.

stimBuffApis/livoxGen1/pcloudStimulusProducer.h

@@ -89,6 +89,7 @@ public:
 	StagingBuffer assemblyBuffer;
 	IoUringAssemblyEngine ioUringAssemblyEngine;
 	StagingBuffer collationBuffer;
+	StagingBuffer averageIntensityBuffer;
 	size_t tempStimulusFrameMem;
 	StimulusFrame tempStimulusFrame;
 	std::atomic<std::shared_ptr<MeshStimulusBuffer>> meshStimulusBuffer;